A method of detecting packet loss in end-to-end communication is described in Non Patent Literature 1. For example, the method described in Non Patent Literature 1 is applied to equipment for measuring the quality of communication between terminals in a communication network connecting many terminals. In the method described in Non Patent Literature 1, a packet capturing section is provided in a path between terminals so that this capturing section will fully capture packets passing through the path. The capturing section has the acknowledgment numbers of the passed packets, and when two or more packets have the same acknowledgment number (i.e., when a duplicate ACK phenomenon has occurred), it detects packet loss.
FIG. 13 is a block diagram showing an example of measuring equipment for detecting packet loss by the method described in Non Patent Literature 1. It is considered, as shown in FIG. 13, a configuration of packet loss measuring equipment 210 to which the method described in Non Patent Literature 1 is applied, which includes a flow identifying means 211, a database (hereinafter abbreviated as DB) 215, a counter/DB reading means 212, a number comparing means 213 and a counter/DB writing means 214. The DB 215 stores, on a flow-by-flow basis, an acknowledgment number in the previous packet in the same flow and a count value indicative of the number of times the acknowledgment number is duplicated. When an individual packet 91 is read, the flow identifying means 211 identifies the flow of the packet. The flow of the packet is identified based on sender and destination IP addresses, send and destination port numbers, a protocol ID and the like. In FIG. 13, each packet indicated by the solid line belongs to the same flow, and each packet indicated by the broken line belongs to another flow.
The counter/DB reading means 212 reads, from the DB 215, the acknowledgment number of the previous packet in the same flow as a new packet identified by the flow identifying means 211 and the count value indicative of the number of times the acknowledgment number is duplicated. The number comparing means 213 compares the acknowledgment number of the new packet identified by the flow identifying means 211 with the acknowledgment number of the previous packet in the same flow. If they match, one is added to the read count value, while if they do not match, the count value is not changed. After this processing, the counter/DB writing means 214 stores, in the DB 215, the acknowledgment number of the new packet and the count value after being subjected to the processing by the number comparing means 213.
Methods of estimating packet loss are also described in Patent Literatures 1 and 2, for example. Further, Patent Literature 3 teaches communication equipment, which refers to a learning table storing port numbers of packets received in the past and a frame storage memory storing the port number of a newly received packet to compare the port number of the newly received packet with those of the past packets. Further, Patent Literature 4 teaches an IP packet counting method for classifying packets based on their port numbers to count the number of packets in a table.